The subject of this study is the elucidation of bbiochemical changes occurring in heteroploid mammalian malignant cells invitro in response to treatment with C21 steroids of the glucocorticoid series. Two culture systems are used. One system, which includes two clones of glass adherent epithelioid human cells (HeLa) shows changes in cellular morphology as well as an altered growth pattern without general toxicity of steroids. In the other cell type, L5178Y mouse lymphoma, the steroid effect is manifested by marked growth inhibition. The biochemical parameters that are being studied relate to physiology of the cell periphery. In HeLa cells they include alterations in proteolytic activity at the cell surface, accumulation of sialopeptides, changes in plasma membrane associated enzymes as well as uptake of nutrients and accumulation of metabolic intermediates. In L5178Y lymphoma, steroid effects studied are concerned with alternate pathways of choline utilization. The aim of this investigation is: (a) understanding, on the biochemical level, of membrane alterations that lead to ordered growth pattern in steroid treated glass adherent cell of neoplastic origin, and (b) search for a site of direct effects of glucocorticoids on lymphatic cells at the level of plasma membrane.